As a result of increasing miniaturization and of the need of ever more efficient energy management, both in terms of processing and engineering, in the design of components and systems (system integration) of chemical reactors, micro-structured reactors, heat exchangers and coolers (micro-structured components) are used both in research and development projects and even start to be utilized in first industrial processes. Micro-structured components generally consist of a stack of thin metal sheets which are characterized by fine structures. Components having very fine channels are obtained by bonding the structured metal sheets. The metal sheets may be structured by dry etching, wet chemical deep etching or by mechanical micro-production.
Usually, the structured metal sheets are provided with a cover and a bottom plate and assembled into a compact component part. If the component parts are constructed accordingly, maximum heat and power exchange may be achieved with minimum component volume, with the flow conditions in the component being selectively adjustable and isothermal conditions being achievable in the micro-channels.
This structure, typically having channel depths in the range of from 50 to 2000 μm, may be produced by dry etching, wet chemical deep etching, laser beam, LIGA methods, spark erosion or by mechanical micro-production for example. The reactors may also be manufactured using methods of the silicon chip technology for example (for bonding the various layers, the typical bonding conditions indicated are 450° C., 750V: anodic bonding). The structured layers of these components, sheet metals for the major part, are finally provided with a cover and a bottom plate and bonded into a compact component part. For the connections of the inlet and outlet region, two or more passageways are preferably etched, punched or lasered into the cover plate.
Micro-structured reactors typically have an inlet region into which reactants enter through an appropriate connection and from which they spread homogeneously over channels from which the products generated in the channels are caused to exit the reactor through an outlet region. The channels may be catalytically coated. The geometry and arrangement of these channels and of the webs between these channels are thereby designed so that fluid dynamics, heat transfer and pressure loss have an optimum ratio with respect to one another, said ratio being defined by the respective application. The inner structure of the components allows for maximum heat and power exchange on minimum surface area, while the flow conditions are specifically adjustable and high isothermy prevails in the channels. This isothermy is a major prerequisite in avoiding, inside the fluid channels, what are referred to as hot and cold spots, which may adversely affect the activity or even deactivate applied catalysts. Beside avoiding hot and cold spots, the possibility of a more precise temperature control during reactor operation as compared to conventional reactors also results in suppressing undesired side reactions, in increasing the selectivity or the product yield and finally in considerably increasing the profitability of chemical reactions. In addition thereto, deposits, such as those caused by carbonization, onto a catalyst system may be avoided by this temperature control.
Moreover, fast heat dissipation in micro-structured reactors may ensure increased security standards. Strong exothermic reactions may thus be controlled—even within the explosion limits. Further, by virtue of the short thermal response times, the reactors systems described are particularly suited for dynamic processes which require fast changing reaction conditions such as flow rate and temperature.
More than 90% of all chemical industrial production processes are carried out with catalysts (J. M. Thomas, W. J. Thomas, “Principles and Practice of Heterogeneous Catalysis”, Verlag Chemie, Weinheim, 1997). This applies in particular for gas phase reactions. In view of this huge number, the problem, which has not yet been solved in practice, consists in finding a way to successfully run catalyzed reactions in micro-structured reactors on an industrial scale.
For loading conventional reactors, catalytically active pellets, balls, Raschig rings, Berl saddles, powder and so on, which are introduced in bulk into the catalyst bed of a solid bed reactor, are commercially available. For reactions with a particularly short dwell time, catalyst meshes, mostly noble metal meshes, are utilized. These forms of commercializing coatings and catalysts cannot, or only conditionally, be used for micro-structured reactors. This is due for example to the risk of clogging of the channels, the increased pressure drop, the inhomogeneous flow profile, the inhomogeneous concentration and temperature profiles resulting therefrom as well as the appearance of the hot/cold spots, which are all due to the form given to the catalyst. This is the reason why micro-structured reactors are, inter alia, coated using what is referred to as wash-coat (slurry coating) or the sol-gel method in order to form particularly thin catalyst layers by physical vapor deposition (PVD) or chemical vapor deposition (CVD), by wet impregnation or precipitation (W. Ehrfeld, V. Hessel, H. Löwe “Microreactors”, WILEY-VCH Verlag, 2000). All the coating methods rely on only loading the reaction channels with catalyst after the reactor has been bonded, this process being referred to herein after as post-coat, meaning that the catalyst is introduced into the channels only upon completion of the reactor, inclusive of the mounting of the inlet and outlet connections. The viscosity of the suspensions and emulsions used hereby limits the minimum diameter needed for the channels of the reactors used in order to prevent them from clogging. The amount and the distribution homogeneity of the catalyst applied can no longer be subjected to non-destructive control once the reactor has been mounted. The inlet and outlet regions are thus also contaminated with the catalyst or even completely coated so that reactions occur prematurely or continue to occur outside of the structured reactor core. This may result in poorer temperature control and, as a result thereof, in poorer reaction control, in undesired side reactions and in reduced conversion and reduced selectivity. To a large extent, the advantages of the micro-structure technology are thus lost.
Another reason why the catalyst was to be introduced into the reactor after carrying out the bonding process is that the techniques for finally, irreversibly joining (bonding) the reactors used for all the methods of manufacturing micro-structured reactors hitherto produced were techniques in which the catalyst would have been otherwise destroyed or seriously affected. The only alternative found was not to bond but to screw the reactor together. On the laboratory scale, micro-structured reactors made from a solid housing into which loosely stacked or mechanically carried foils are introduced together with the catalyst are utilized. Usually, these reactors are mounted by mechanical screwing. One advantage is that the discrete foils can be readily replaced without destroying the housing. Major disadvantages are the risk of leakage, which involves potential leakage of hazardous substances as well as poorer thermal coupling. For this reason, seals are needed in reactors bonded this way, said seals significantly limiting the applicable temperature range. Additionally, thermal coupling and, as a result thereof, heat distribution in full metal (not screwed) reactors and, hence, process control are significantly improved.
For the reasons mentioned, various attempts have been made to introduce the catalyst into the reaction channels before assembly. Such a method is referred to as pre-coat, in contrast to post-coat.
The pre-coating method for loading catalysts into a reactor allows for precise quality control of the applied catalyst layers with regard to coating thickness, amount of catalyst, homogeneity and site of deposition. It moreover prevents contamination of the inlet and outlet regions through the catalyst. For the first time, the micro-structure may be loaded with temperature-sensitive catalyst if a suited bonding process is utilized. This opens a wide field of heterogeneous catalyst applications to the micro-structure technology. The prerequisite for the utilization of temperature-sensitive catalysts is a bonding temperature adapted to the temperature stability of the catalyst, while it must be ensured that the temperature stability of the reactor is sufficient to carry out the chemical conversion.
However, all the methods of bonding micro-structured reactors presently preferably used in practice do not allow for a pre-coat method for applying the catalyst because of the process conditions and/or the additives used (e.g., fluxing agents):
Diffusion welding requires a high bonding temperature (of 1000° C. for example), a high pressure as well as a good vacuum during bonding. A good vacuum is used to minimize the formation of oxides at the surface of the components to be bonded since oxide layers would considerably affect successful bonding. Another disadvantage of this method is the long holding and processing time. The high temperature required makes the use of diffusion welding for pre-coat temperature-sensitive catalysts totally inadequate. According to current state of knowledge, even acknowledged high temperature catalysts are deactivated or their activity is considerably affected under these process conditions.
DE 198 25 102 C2 describes a method of manufacturing a compact catalytic reactor. This method comprises catalyst application before bonding and soldering for bonding the component. As contrasted to the method of the invention, DE 198 25 102 C2 does not teach to apply the bonding layer in the passageways or on the webs. The lack of bond across the webs generally leads to poorer thermal coupling of the various reactor layers. The interrupted heat conduction between the reactor layers prevents isothermy, which is a major method advantage in full-metal micro-structured components, thus leading to minimized temperature control of the reactions within the reactor. Transverse leaks may further appear between the various passageways, such leaks leading to undesired mixtures and reactions. This may happen at the expense of selectivity and yield. In order to ensure tightness from outside, the borders are merely bent before the various reactor layers are stacked onto each other. This is necessary since the bonding process described in DE 198 25 102 C2, which uses solder foils, does not ensure a gas-tight bond. Moreover, the use of solder foils may cause clogging of the passageways.
Adhesive bonds do not sufficiently meet the required temperature resistance and significantly affect the thermal conductivity of the components. Furthermore, additives, solvents or the adhesive bond itself interact with the catalysts used. Further, the risk that the channels become clogged by inhomogeneous application of an adhesive is very high.
Soft soldering as a thermal bonding method carried out in vacuum or in an inert gas atmosphere, is utilized on a large scale. The solder foils or pastes used thereby on the micro-structure cause the channels to become clogged so that they are not suited for use in bonding micro-structured components. Further, the addition of fluxing agents may cause corrosion because of its accumulation in the solder gap of the microchannels, too short a temperature profile or a wrong chemical composition of the fluxing agent causes the formation of cavities and moreover involves high environmental impact that may only be minimized by complex and cost-intensive waste water and air purification. Furthermore, undesired reactions between the fluxing agent and alloy additives of the base material may occur, thus preventing successful bonding. The use of fluxing agents in the manufacturing of catalytically coated reactors may deactivate the catalyst.
Beside the advantages mentioned above regarding reaction control and high security standards, a micro-structured component should meet the following important technical requirements:
1. Sufficient tightness, both between the channels and against the surroundings;
2. Sufficient pressure resistance or strength;
3. Sufficient corrosion resistance against the media used;
4. Sufficient temperature resistance;
5. Free, geometrically homogeneous fluid channels.
The major demands placed on the manufacturing method may be summarized as follows:
1. High flexibility and adaptability to the overall system or the peripheral geometries;
2. High flexibility in the design according to the given specification;
3. Scalability to low-cost industrial mass production;
4. The bonding method must allow for the possibility of the pre-coat;
5. The activity of the catalysts must not be affected.
Hitherto, there is no low-cost method suited for industrial scale for manufacturing catalytically coated micro-reactors for low temperature and high temperature applications that would meet all of these requirements. Hitherto, micro-structured reactors have been almost exclusively utilized to carry out non-catalytic reactions because there was no convincing technique for coating the channels that could be used together with the method of manufacturing the entire component.
A gentle method of assembling micro-structured component layers suited for manufacturing micro-structured components has been described in EP 1 415 748 A2. This documents mentions, inter alia, the melt diffusion method. This method is understood to refer to a soldering method in which several elements of the solder interdiffuse, thereby forming intermetallic phases. The composition and thickness of partial solder layers may be for example matched in such a manner than an initial eutectic forms during bonding. Accordingly, a very low melting temperature is achieved at the beginning. With interdiffusion of the solder elements between various partial solder layers, the melting point progressively shifts to a higher value during the soldering process. By tempering the bond, a solid solder bond is progressively obtained with this method, said solder bond having a melting point that is considerably higher than the initial melting point when the solder layer starts melting. As a result, soldering may occur at a very low soldering temperature. This in particular allows very gentle processing of the various component layers so that warping of the discrete layers when subjected to thermal load may be practically excluded.
Further, EP 1 198 344 B1 indicates a method of manufacturing micro-components in which a catalyst is applied to the channel walls prior to bonding the various component layers. According to this document, channels are formed first. For this purpose, a copper foil is coated with a structuring cover layer (a photoresist layer, a screen printing lacquer layer, a perforated foil or a metal resist layer). The channel areas are thereby left uncoated or are exposed. The copper foil is etched in the bare areas, e.g., with a FeCl3/HCl solution, so that recesses corresponding to the channels to be formed are formed in these areas. Next, catalyst is formed on the channel walls only. The catalyst in the channels is thereafter coated with another cover layer. Subsequent thereto, the cover layer is selectively removed. Then, a bonding layer may be deposited in the exposed areas. After the cover layer has been removed from the channels, several layers made in this way are joined by soldering. It has been found that the efficiency of the catalyst during manufacturing is considerably reduced if not completely eliminated.
Accordingly, it is the object of the present invention to eliminate the disadvantages of the prior art methods and more specifically to find a method of manufacturing micro-structured reactors that makes it possible to manufacture micro-structured reactors loaded with a catalyst in compliance with the technical requirements and the production method. In this way, micro-structured reactors may also be utilized in heterogeneously catalyzed reactions. Moreover, the manufacturing method must offer the possibility of low-cost industrial conversion for manufacturing micro-structured components. It finally aims at finding a method of manufacturing micro-structured reactors that may be utilized as hydrocarbon and alcohol reformers, in particular as methane and methanol reformers. The reactors thus produced should be small, light in weight and compact for varied and above all mobile applications in particular. The manufacturing method should be automatable and scalable. Eventually, the method should serve to manufacture reactors having a high WHSV (weight hourly spatial velocity: amount of converted material [g] in the reactor per catalyst mass [g]×operating time of the reactor [h]).